Laser generator of te wave guide modes

ABSTRACT

Propagating radiation with rotationally symmetric optical modes of the TE01 (and TM01) type show promising properties for future applications. For their generation, laser cavities inherently enhance the rotationally symmetric TEM*01 (and TEM*10) laser modes. These cavities must suppress the otherwise usual fundamental TEM00 mode, and in addition discriminate against unwanted same order modes like the linearly polarized TEM01 and TEM10 laser modes. A birefringent crystal zero degree calcite inserted into the cavity applies higher losses at the aperture to the extraordinary beam showing the TEM*10 mode. Thus, the ordinary beam showing the wanted TEM*01 laser mode is favored. Mode selection is further assisted by a Galilean telescope-like setup of the crystal. Discrimination against fundamental mode and higher order modes may be improved by the insertion of circular or ring shaped apertures in the telescope arrangement.

United States Patent [1 1 Pohl [111 I 3,777,280 Dec. 4, 1973 [75]Inventor:

[ LASER GENERATOR OF TE WAVE GUIDE MODES Dieter Pohl, Adliswil,Switzerland [73] Assignee: International Business Machines Corporation,Armonk, NY.

[22] Filed: June 21, 1972 [21] App]. No.: 265,014

[30] Foreign Application Priority Data Aug. 12,1971 Sweden ..ll89l/71[52] US. Cl. 331/945 [51] Int. Cl. H015 3/10 [58] Field of Search331/945 [56] References Cited UNITED STATES PATENTS 3,435,371 3/1969White 331/945 3,628,173 12/1971 Danielmeyer 331/945 PrimaryExaminerWilliam L. Sikes Att0rneyGeorge Baron et al.

57 ABSTRACT Propagating radiation with rotationally symmetric opticalmodes of the TE (and TM type show promising properties for futureapplications. For their generation, laser cavities inherently enhancethe rotationally symmetric TEM* (and TEM* laser modes. These cavitiesmust suppress the otherwise usual fundamental TEM mode, and in additiondiscriminate against unwanted same order modes like the linearlypolarized TEM and TEM laser modes.

19 Claims, 4 Drawing Figures Pmmmm ms FIG. 2

FIG. 3

1 LASER GENERATOR OF TE WAVE GUIDE MODES BACKGROUND OF THE INVENTION Theinvention relates to a laser for the generation of light in rotationalsymmetricmodes, especially the TE propagation mode.

Common laser oscillators for the generation of stimulated monochromaticand coherent radiation emit linearly polarized light. The laserresonator is normally excited in the simplest mode with the lowestlosses, the so-called ground mode. In comparison with the wavelength ofthe radiation, the dimensions of optical cavities are large. Since foreach standing wave the length of the cavity must equal an even multipleof the half wave length many resonator modes are possible. In theresonator every existent standing wave represents a socalled mode whichcan be distinguished by its polarization, direction and frequency.

For controlling the frequency of the radiation, longitudinal modeselectors are known which maintain constant the number of half wavesover the length of the cavity. However, besides each longitudinal modethere exists a considerable number of so-called transverse modes havingall approximately or exactly the same frequency but different fielddistribution in the transverse direction. They can be defined after thenumber of their node lines which lines may be seen for instance in theradiation pattern of the laser or on the reflectors terminating thelaser cavity. Linearly polarized modes show orthogonal node planes.These modes are designated as TEM laser modes with double indiceswhereby these indices define the number of node planes. Hence TEMdesignates the ground mode having no node planes. Higher transversemodes show node areas according to the indices, like TEM- For the sakeof clarity, rotational symmetric laser modes are in addition designatedwith an asterisk in the following way. In this way, the TEM* laser modedesignates'a mode having a central node cylinder.

Rotational symmetric laser modes are of growing importance since theylead to the generation of optical radiation in the TE or TMm propagationmode, respectively. These are the two lowest order rotational symmetricpropagation modes as we know from micro wave technology. Out of thesetwo, the azimuthally polarized radiation of the TE mode looks especiallypromising in view of unique properties. When the technology ofinformation transmission is expanded from micro-waves to the region ofoptical frequencies this wave type, namely, TE can be transmittedwithout loss and with no sensitivity against bends of the metallic waveguide. TE, and TM, are the lowest order propagation modes for whichdielectric wave guides have a defined and finite cut-off frequency.Feeder connections to rotating antenna systems can easily be built dueto the rotational symmetry of the wave types involved. In addition,interesting non-linear optical effects are to be expected as can beestimated from calculations. Among the low order propagation modes onlythe TE wave exhibits the property of self-trapping, that means, it canbuild its own wave guide as a filament in certain media.

The discrimination between degenerate same order modes is especiallydifficult for lasers which should operate in pure rotational symmetricmodes and which should discriminate against the ground mode. Suchdegenerate modes are known for having the same frequency and the sameenergy distribution over the beam cross-section. For this reason theycannot be separated, e.g., by apertures, without applying further means.

In accordance with the teaching of US. Pat. No. 3,283,262, a laser hasbecome known for the generation of TM mode radiation whose transversemode selector comprises at least one conical dielectric interface. Theapex of the cone is located in the optical axis and the cones angle ofthe glass cone is equal to the complementary-Brewsters angle. Hence theradially polarized TM wave is favored among the rotational symmetricpropagation modes of the radiation. However, the manufacture of thisknown device demands the preparation of optically accurate conicalsurfaces. This kind of production method is, however, especiallydifficult. Only plane or spherical surfaces can be manufactured inoptically useable quality at reasonable cost.

OBJECTS OF THE INVENTION It is therefore the object of the invention todesign a laser for the generation of light in rotational symmetricmodes, especially in the TE propagation mode, comprising a suitable modeselector which can be easily manufactured. According to the invention,the laser is characterized in that, within the optical cavity,transverse mode selection means are provided which comprise at least onebirefringent uniaxial crystal for the discrimination between degeneratehigher order modes whereby the crystal optical axis coincides with theoptical axis of the cavity.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows schematically the generalset-up of a laser for the generation of light in rotational symmetricmodes.

FIG. 2 is used for the explanation of the TE propagation mode.

FIG. 3. is used for the explanation of the TM propagation mode.

FIG. 4 shows schematically the transverse mode selector to be used inthe optical cavity of the laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As an example, there isshown inFIG. l a solid state laser with a passive Q-switch for thegeneration of giant pulses. The active medium of the laser is a rubycrystal 1, adjacent to it there is arranged the pump light source 2. Theopticalqavity is terminated by reflectors 3, 4. One of'the reflectors 3is suitably designed as a resonant reflector for longitudinal modeselection. The other reflector 4 serves also as an output mirror. In theradiation path there is provided a Q-switch 5, e.g., a

cell containing a bleachable dye-like cryptocyanine dissolved inmethanol, This passive Q-switch 5 is shown in the drawing between thesolid'state active medium 1 of the laser and the terminating reflector3. It can be inserted also at another section of the radiation beam, forexample, adjacent to the transverse mode selector 6. Particulars of thistransverse mode selector 6 are explained hereinafter in connection withFIG. 4.

FIGS. 2 and 3 show the rotational symmetric field distribution of themost interesting radiation propagation modes TE and TM At the bottom ofthe per-. spective views an arrow indicates the propagation direction.As can be seen from FIG. 2, the closed transverse electrical field linesof the TE mode enclose orthogonally the propagation coordinate directionin a ring-shaped manner. One can clearly perceive from the plan view theazimuthal polarization of this mode resulting in a radiation patternlike a bright tube with a dark kernel. The orthogonal dashed magneticfieldlines run radially in planes perpendicular to the propagationcoordinate direction.

In the TM propagation mode the transverse magnetic field-lines areclosed rings perpendicular to the axis. As can be seen from FIG. 3, theelectric field-lines form closed loops within planes of a plane bundlewhose planes all contain the propagation coordinate direction. The planview in propagation direction shows the radial polarization of thetransverse magnetic TM. mode by the entered arrows. Both types ofrotational symmetric propagation are polarized perpendicularly to eachother. Hence, they can be converted into each other by a 90 opticalactive medium. A quartz crystal, rotating the polarization direction andinserted into the radiation path, converts an azimuthal polarizationinto the radial polarization and vice versa. Thus both rotationalsymmetric propagation modes of the radiation can be generated providedthat only one of both modes exists purely.

FIG. 4 shows schematically a transverse mode selector whoseincorporation into the optical cavity enables the laser to generatelight of the TE mode. This type of mode selector is suitable for everykind of laser be it a solid state laser or a gas laser. A continuouswave gas laser is advantageous for information transmission application.The embodiment described relates to a giant pulse laser with a rubycrystal as the active medium.

The transverse mode selector has to fulfil the following tasks. Theground mode TEM without any node plane which can be excited in theeasiest way should be suppressed. Also higher order laser modes havingmore than one node area should not be excited. The most difficult task,however, is the discriminationof the desirable rotational symmetric TEM*laser mode with one node cylinder leading to the generation of the TEpropagation mode optical radiation against other excitable same ordermodes. To this group of modes belongs the other rotational symmetricTEM* laser mode with the same intensity distribution which leads to thegeneration of TM propagation mode optical radiation, or the linearlypolarized TEM and TEM laser modes having each one node plane or othermixed types or superpositions of same order laser modes. All these modesare degenerate, i.e., they have the same frequency and the same energydistribution across the lower order lowest higher order modes isfavored. This group of modes, however, is of similar transverseextension. Therefore one cannot achieve a separation of the modes by theinsertion of a diaphragm into the beam path. Particularly the earliermentioned rotational symmetric laser modes TEMH and TEM'N are completelydegenerate, i.e., their respective intensity distributions across thebeam cross-section are identical.

To lift their degeneracy and to make them distinguishable, one lets themcross a uniaxial birefringent crystal rotated symmetrically with respectto its optical axis direction. The orthogonally polarized beams withazimuthal and radial polarization are each diverted in a differentmanner due to the crystals birefringency. To use this effect best,additional optical means should be provided to make the laser beam asdivergent as possible within the birefringent crystal. By way ofexample, a suitable arrangement of the birefringent crystal is usedwithin a telescopic system like a Galilean telescope or like anastronomical telescope. Such a device providing a telescopic ray bundlecan be inserted in the plane optical cavity of the Fabry Perot type atany time. It is, however, possible to build spherical or hemisphericallaser resonators in such a way that the birefringent crystal is locatedin a region having a divergent ray bundle. For this purpose at least oneof the fierminating reflectors of the optical cavity must be c rved.

According to FIG. 4, a birefringent crystal 10 of calcite (CaCo isprovided in the optical axis 11 of the resonator, which crystal isshaped as a cylinder with plane front faces. The telescopic systemconsists of a first biconcave lens 12 and a second biconvex lens 13.Between the terminating reflector 14 of the resonator and the secondlens 13, i.e., at the side of the larger beam cross-section, a circulardiaphragm 15 is provided. It serves for the suppression of higher ordertransverse modes of the laser light by aperture losses and also for thesuppression of the TM propagation mode made more divergent with thebirefringent crystal by favoring the TE propagation mode. A centralcircular stop 16 on a transparent carrier 17 in the optical axis servesfor screening the ground mode of the laser light.

The laser light being made divergent within the birefringent crystalundergoes a small but distinct double refraction. With regard to theordinary ray w, Snell's law of refraction is valid in its known form(sin a/sin B), n

(sin a/sin B), n (1 3 /2 n, n ln Round figures of the indices ofrefraction in calcite are n, 1.7 for the ordinary ray and n, l.5 for theextraordinary ray. Due to the difierent refraction 'values, the diameterof the diverging extraordinary ray becomes larger than that of theordinary ray.

Within the birefringent crystal 10, the E-Vector of the ordinary ray isdirected azimuthally and the E- Vector of the extraordinary rayradially. Hence the differently polarized rotational symmetric lasermodes spread differently within the crystal. The lowest ordinary mode isthe desired TEM* laser mode for the generation of TE mode light. Thelowest extraordi-' nary mode is the TEM* laser mode to be suppressed.

Due to ray broadening, the latter undergoes higher losses by thecircular diaphragm 15 or the aperture stop 16 of the optical system.Thus, it is quite possible to choose the dimensions of the mode selectorin such a way that already the aperture stop of that optical systemprovides the suitable diameter of the higher order mode diaphragm whichotherwise must be inserted as circular aperture. Therefore, a separatecircular aperture can be omitted by proper design. It has shown thatwith proper design possibly even the circular screen used to eliminatethe ground mode can also be omitted. To guarantee a reliable operationof the mode selector, it is recommended, however, that one combine bothmode diaphragms practically in a ring-shaped aperture.

The different indices of refraction for the ordinary ray and for theextraordinary ray effect also a lifting of the frequency degeneracy,i.e., they take effect in the sense of a frequency separation orselection. Because this difference is, however, very small, it isrecommended that the laser be operated close to the border of stabilityof the resonator. This happens in a resonator with plane mirrors whenboth foci F l and F2 of the lenses L1 and-L2 of the telescopic systemcoincide. Tests have shown, however, that it is practical to provide asmall distanceof those foci designated 18 in FIG. 4. This distance 18was about 0.5 mm in a mode selector whose first lense 12 had the focaldistance fl 2 cm and whose second lens 13 the focal distancej2 =+6 cm.The length of the calcite crystal was 4 cm. The diameter of the circularaperture 15 was 5 mm and that of the central circular screen stop 16 was0.7 mm. The

mode selector 6 was insertedin the optical cavity of a giant pulse laserof about 60 cm length. One of the plane mirrors was highly reflecting,the second terminating reflector of the cavity was a resonant reflectorfor longitudinal mode selection. The active medium 1 of the solid statelaser was a 0 ruby crystal of 7.5 cm. A passive Q-switch 5 withcrypto-cyanine dissolved in methanol caused giant pulse laser operation.The pulses were of about 50 nsec halfwidth with a peak power of 100 300kW.

The laser crystal, i.e., the active medium 1, can be located on eitherside of the mode selector 6. At the larger beam diameter side also themode volume is larger in the active medium resulting in more intenselight pulses. Placing the active medium onthe lower beam diameter sideresults possibly in a better radiation pattern if the ruby crystal 1includes inhomogeneities. The Q-switch 5 can be arranged at any placewithin the cavity.

To make the light divergent within the birefringent crystal 10 also,lenses can be applied with one plane surface adjacent to a planefrontface of the crystal 10. However, the frontfaces of the crystal 10which are shown in the embodiment as plane faces can also be formed asspherical faces which operate as lenses at the same time. One shouldavoid disturbing reflections by spherical faces causing possibly anundesired focus within the crystal 10 affecting it by local destructionwith the powerful laser radiation.

I claim:

l. A laser having an optical cavity and an active medium for thegeneration of light in totally rotational symmetric modes havingrotationally symmetric polarization as well as intensity polarization,especially the TE propagation mode, characterized in that within theoptical cavity of said laser, having terminating reflectors, there areprovided transverse mode selection means including, for thediscrimination between degenerate higher order modes, at least onebirefringent uniaxial crystal whose optical axis coincides with theoptical axis of the cavity.

2. The laser of claim 1 wherein said birefringent crystal consists ofcalcite (CaCO;).

3. The laser of claim 2 characterized in that said birefringent crystalhas the shape of a cylinder with plane front faces.

4. The laser of claim 3 including optical means for causing the laserbeam to be divergent within the birefringent crystal.

5. The laser of claim 3 characterized in that the birefringent crystalis arranged in a telescopic lens system between a diverging and aconverging lens of the kind of a Galilean telescope, the axis of 'saidtelescopic lens system coinciding with the axis of said laser.

6. The laser of claim 5 characterized in that the telescopic lens systemhas coincident foci.

7. The laser of claim 5 characterized in that both foci of thetelescopic lens system are separated by a small distance.

8. The laser of claim 3 characterized in that the birefringent crystalis arranged in a telescopic lens system between converging lenses of thekind of an astronomic telescope, wherein one end of said telescopic lenssystem has a large laser beam diameter side and the other end of saidtelescopic system has a small beam diameter side.

9. The laser of claim 8 characterized in that for the transverse highermode suppression of the laser light by aperture losses, a circularaperture is arranged between the birefringent crystal and one of saidterminating reflectors.

10. The laser of claim 8 characterized in that a central circular screenis provided at the optical axis to eliminate the ground mode ofpropagation of the laser light.

11. The laser of claim 10 characterized in that there is provided aring-shaped diaphragm for the mode selection.

12. The laser of claim 8 characterized in that the active medium of thelaser is arranged in the cavity before the birefringent crystal at thesmaller beam diameter side.

. 13. The laser of claim'8 characterized in that the active medium ofthe laser is arranged in the cavity after the birefringent crystal atthe larger beam diameter side.

14. The laser of claim 1 characterized in that the active medium of thelaser is a solid state body.

15. The laser of claim 1 characterized in that the active medium is aruby crystal..

16. The laser of claim 1 characterized in that the active medium of thelaser is a glass doped with ions of a transition metal.

17. The laser of claim 1 characterized in that the active medium of thelaser is a gas or a mixture of gases.

18. The laser of claim 1 characterized in that the laser is a continuouswave laser. 7

19. The laser of claim 1 characterized in that the laser-is-a Q-switchedgiant pulse laser.

1. A laser having an optical cavity and an active medium for thegeneration of light in totally rotational symmetric modes havingrotationally symmetric polarization as well as intensity polarization,especially the TE01 propagation mode, characterized in that within theoptical cavity of said laser, having terminating reflectors, there areprovided transverse mode selection means including, for thediscrimination between degenerate higher order modes, at least onebirefringent uniaxial crystal whose optical axis coincides with theoptical axis of the cavity.
 2. The laser of claim 1 wherein saidbirefringent crystal consists of calcite (CaCO3).
 3. The laser of claim2 characterized in that said birefringent crystal has the shape of acylinder with plane front faces.
 4. The laser of claim 3 includingoptical means for causing the laser beam to be divergent within thebirefringent crystal.
 5. The laser of claim 3 characterized in that thebirefringent crystal is arranged in a telesCopic lens system between adiverging and a converging lens of the kind of a Galilean telescope, theaxis of said telescopic lens system coinciding with the axis of saidlaser.
 6. The laser of claim 5 characterized in that the telescopic lenssystem has coincident foci.
 7. The laser of claim 5 characterized inthat both foci of the telescopic lens system are separated by a smalldistance.
 8. The laser of claim 3 characterized in that the birefringentcrystal is arranged in a telescopic lens system between converginglenses of the kind of an astronomic telescope, wherein one end of saidtelescopic lens system has a large laser beam diameter side and theother end of said telescopic system has a small beam diameter side. 9.The laser of claim 8 characterized in that for the transverse highermode suppression of the laser light by aperture losses, a circularaperture is arranged between the birefringent crystal and one of saidterminating reflectors.
 10. The laser of claim 8 characterized in that acentral circular screen is provided at the optical axis to eliminate theground mode of propagation of the laser light.
 11. The laser of claim 10characterized in that there is provided a ring-shaped diaphragm for themode selection.
 12. The laser of claim 8 characterized in that theactive medium of the laser is arranged in the cavity before thebirefringent crystal at the smaller beam diameter side.
 13. The laser ofclaim 8 characterized in that the active medium of the laser is arrangedin the cavity after the birefringent crystal at the larger beam diameterside.
 14. The laser of claim 1 characterized in that the active mediumof the laser is a solid state body.
 15. The laser of claim 1characterized in that the active medium is a ruby crystal.
 16. The laserof claim 1 characterized in that the active medium of the laser is aglass doped with ions of a transition metal.
 17. The laser of claim 1characterized in that the active medium of the laser is a gas or amixture of gases.
 18. The laser of claim 1 characterized in that thelaser is a continuous wave laser.
 19. The laser of claim 1 characterizedin that the laser is a Q-switched giant pulse laser.